1. Field of the Invention
The present invention relates to a Global Positioning System (GPS) and a method for a moving station in a local area. More particularly, this GPS system relates to the technology of a real-time self Differential Global Positioning System (DGPS) and a method for accurately positioning a moving station that is located within a radius of several kilometers, and applying the technology to an industrial terminal operation.
2. Description of the Prior Art
In recent decades, many cutting edge technologies have been competitively developed to lead the world markets. One of these technologies is the Global Positioning System (GPS) and Method that is able to determine a three-dimensional position and time anywhere on or near the surface of the Earth for a moving station. The GPS system employing at least four satellites is able to position a moving station by receiving satellite-based navigational data of location, speed and time through receivers. However, the conventional system has a wide error margin of approximately 100 meters, which is not acceptable in reality. The large margin of error is due to the GPS (non-military GPS) orbit, delay from passing through the Ionosphere and Troposphere and Selective Availability (SA).
Many sources of error can be compensated for by using a Differential GPS (DGPS) system, which can improve accuracy to within about 5 meters. As shown in FIG. 1, a typical configuration of a conventional DGPS system illustrates a navigation system capable of positioning a client through the following process: a reference station of known position, (1) calculates an error range in a pseudo distance according to the data received from a satellite, (2) transmits the calculated error range to the nearby user, and (3) determines the real position. Next, (4) the DGPS system broadcasts the correcting message via a broadcasting system. The correcting message for correcting a position error is calculated according to the instantaneous comparison of the pre-measured position at the reference station with the calculated position according to the received GPS signals. For broadcasting the correcting message, the correcting message is transformed to a standard format of Radio Technical Commission for Maritime Services (RTCM). The transformed correcting message is modulated to the ultrahigh frequency by a modulator (5), transmitted through a transmitter (6), and received through a receiver (7) for reflecting on the calculation of the positions.
The errors contained in the GPS signals (i.e., errors caused from the GPS satellite orbit, GPS satellite time, delay of Ionosphere and Troposphere when the GPS signals pass through these layers, interference of multi-channel and receiver noise) are considered as common errors between the reference station and the user, excepting the interference of multi-channel error and receiver noise error. In order to avoid or minimize these sorts of errors, a process is adopted based on the theory of decay.
Moreover, the published Korean Patent Application No. 1999-15845 and the registered Korean Patent No. 260253, disclose that: the benefits of the error compensation performed by DGPS decrease or disappear entirely as the distance between the DGPS reference station and the user increases (normally more than 100 Km), because the common errors have the characteristics of decay. To reduce or limit the occurrence of common errors, a plurality of reference stations must be established. A technique of minimizing error range is developed by properly modifying an error correction being calculated by more than 2 reference stations within a certain distance.
As shown in FIG. 2, a flow chart of the conventional DGPS performance is presented for positioning a client by adopting a plurality of reference stations. In step S0, user""s pseudo distance including error is determined according to the GPS signals. In step S1, the correction values of pseudo distance (PRCi) are received from each DGPS reference station. In step S2, among the received correction values of pseudo distances between the reference stations and users, only those values received from within 100 Kilometers are selected, and the rest of the correction values are discarded. In step S3, calculate a space functional correlation ratio (Pwiu) between a reference station and user and a space functional correlation ratio (Pwiwj) between the reference stations. Step S4 determines a modifying value (Wi) for applying to the error correction from the i-th reference station. Step S5 calculates the user""s correction value (PRCm) of pseudo distance according to the modifying value (Wi) determined in the previous step 4 and the correction value of pseudo distance (PRCi) received from each reference station. Step S6 accurately calculates a final correction value (PRC) of pseudo distance by adding or subtracting the correction value of each reference station to the user""s correction value of the pseudo distance calculated from each DGPS receiver. Step 7 determines an accurate final user""s position according to the final correction value (PRC) and the user""s pseudo distance (including error) determined from the GPS signal (S0).
Application of the DGPS system requires installing expensive DGPS receivers and additional transmitting equipment. Therefore, this system is not suitable to operate a plurality of moving stations due to the requirement of installing the expensive equipment. Another system of Post Processing DGPS data has been suggested to reduce the errors and operating cost. However, this system has difficulty to apply to real time DGPS.
In order to overcome the problems of the conventional GPS and DGPS as discussed above, a unique real-time self-Differential Global Positioning System (DGPS) of the present invention is developed for positioning moving stations in real time through a wireless modem within a local area with a radius of several kilometers (desirably in the range of 1xcx9c2 Kilometers). This new system enables not only to calculate, transmit and receive the correcting values via only the GPS receiver, but also to reduce error ranges to several decimeters or several centimeters, which is the same level as the DGPS, without necessitating additional equipment.
The real-time self DGPS system for a moving station is operated under the circumstance that a client communicates to a server through a wireless LAN network in a local area. The real-time self DGPS system comprises: a private reference station (10) which has had its position predetermined by an accurate measurement and includes a reference station GPS receiver (11) for positioning via satellites.
A moving station (30) includes a moving station GPS receiver (31) for detecting position via satellites and a first wireless modem (32) for transmitting RGPS (real time GPS)? signals. The RGPS signals contain the position data and satellite time data. The moving station GPS receiver (31) is connected to the first wireless modem (32) to transmit the signals.
A central control unit (40) comprises a terminal GPS (TGPS) server(41), a central control server (42) and a second wireless modem (43). The second wireless modem (43) is connected to the first wireless modem (32) of the moving station (30) for communicating with each other over a relatively short distance.
The TGPS server (41) is interfaced with the reference station GPS receiver (11) to receive the RGPS signals from the reference station. The TGPS server (41) calculates correction data (Xxe2x80x20, Yxe2x80x20) from the received data and transmits the calculated correction data to a central control server (42).
The central control server (42) interfaces with the second wireless modem (43) which receives the RGPS signals from the moving station. The central control server (42) calculates the real position of the moving station by correcting the error from the position RGPS signals received from the moving station. The position data is transformed to the coordinate values of the local area. The local area is defined within a radius of several kilometers.
An industrial Terminal Global Positioning System (TGPS) employing a real-time self DGPS in a local area comprises: the position data being received through the existing wireless modem used for controlling and manipulating job control data of the moving station without any additional transmitting-receiving equipment. The code of the position data and job control data is composed of a string including a distinguishing header.
A method of real-time self DGPS comprises the processing steps of: (S10) at a specific moment, the central control server (42) receives a moving station GPS signal (Mgps_0) containing the data of satellite time (To) and moving station position (Xm0, Ym0) measured by a moving station GPS receiver (MGPS) (31); (S12)The received data of satellite time (To) of the moving station GPS signal is transmitted to a TGPS server (41)); (S13) receiving the reference station GPS signal (Rgps_xe2x88x922, Rgps_xe2x88x921, Rgps_0, Rgps_1, Rgps_2) containing satellite time (To) data and position data (Xr, Yr) being measured by the reference station GPS (RGPS) receiver (11) located at the private reference station (10) close to the satellite time (To) in TGPS server (41); (S14) selecting a position data (Xr0, Yr0) of the reference station GPS signal corresponding to the satellite time (To); (S20) calculating a correcting value (Xxe2x80x20, Yxe2x80x20) according to the selected reference station GPS signal (Rgps_0) and the stored actual position data (Xs, Ys) of the reference station; and (S24) calculating an error-corrected actual position data (Xcm0, Ycm0) of the moving station according to the correcting value and the position data of the moving station.
Generally, the error ranges of the conventional GPS and DGPS systems are in proportion to the distance between the reference stations. To increase the accuracy of a DGPS system, the error must be reduced to an acceptable range of several decimeters or several centimeters. To do so, the additional equipment required for modifying the conventional GPS or DGPS system is costly.
On the other hand, the DGPS system of the present invention establishes a private reference station within a several kilometers radius in a relatively narrow local area. Through the existing wireless modems used for controlling and governing, the new DGPS system enables to transmit-receive a correcting value for GPS signals to accurately position the moving stations and reduce the error range without installing any expensive equipment.
Most GPS signal data measured by the conventional technique contains wide and irregular error ranges if the instant when a moving station needs a correcting value is different from the instant when a server transmits the correcting value. Thus, the conventional technique is not reliable because the error range is wide and irregular every time.
Even though the moments of requesting and receiving the correcting value for GPS signal data may be different, the technique of the new DGPS system enables to accurately calculate the correcting values and track the trajectories of the moving stations by using a log and satellite time data transmitted when the moving station requests the correcting value via the TGPS server.